The present invention relates to an output device for simultaneously outputting a proof sheet and a planograph plate in a total printing system by using an electrophotographic recording medium, and to a printing method by using the output device. Prior to the description of the present invention, the term of "a planographic" is defined as a printing plate which is one of various printing plates without ruggedness and in the printing process. The planographic separates picture line portions (printed portions) and non picture line portions (non-printed portions) by a chemical or electrical function. Furthermore, the planographic divides the boundary between portions of one color and portions of another color in a color printing.
In the printing method using the electrophotographic plate, a pattern is formed over the surface of a photosensitive medium including a photoconductive medium and toner which consists of extremely fine and charged particles attached to a charge latent image, thereby forming a visible image.
Various methods for forming the electrophotographic image have been devised and demonstrated. In the CPC process, a toner image is formed on the surface of a recording medium in which a photosensitive medium consisting of as a photoconductive medium coated over a base. Another example is the PPC process in which transferred to a plain paper is a toner image formed over a recording and transfer medium having a photosensitive medium containing a photoconductive medium.
In the case of the CPC process of the type described above, the recording medium itself is photosensitive and a toner image is directly formed over the recording medium by the electrophotographic process, it is possible to easily form an image with a high degree of resolution, but this process has a problem that the color reproduction of a color image is adversely affected by the color of the surface of the recording medium.
On the other hand, according to the PPC process, freely used as a transfer paper is a white paper on which the color reproduction of a color image can be satisfactorily obtained.
However in the case of the PPC process (plain paper copier), even when a toner image formed over the cylindrical surface of a photosensitive drum to be transferred to the surface of a plain paper has a high degree of resolution, in the case of transfer, the image transferred to the plain paper has a relatively degraded degree of resolution. Because of this reason, a satisfactory color image having a high degree of resolution cannot be obtained.
But in the case of the total printing system, prints are obtained through the steps of obtaining an image, edition, preparing a block copy, making of a printing plate and printing. FIG. 18 is a flow chart schematically illustrating the planographic printing process obtaining a color image by the conventional printing system. In this case, the original of a color image to be printed is supplied as a color film. In the case of the printing system shown in FIG. 18, when the original color film is read out by a printing plate camera (for instance, an image scanner or a drum scanner) so as to obtain a colored image or print, in general, the original color is subjected to the color separation so as to be a plurality of colors; that is, the three primary colors for the subtractive mixture--yellow (Y), magenta (M) and cyan (C) in addition to black (BL). When the color separation films, in which each color is deposited at each screened dot, are made, they are then subjected to the proofing step so that a printing plate is made and proofed or the printing of a proof sheet is made based upon these color separated films.
When the printed proof sheet is unacceptable in quality, the color separated films are manually corrected, retouched, edited, patched and so on. Thereafter, they are returned to the original reading step. And these steps are repeated until a satisfactory proof sheet is obtained.
When the satisfactory proof sheet is thus obtained, a planographic printing plate is formed based on the color separated films for the actual printing and the actual printing is started.
In the printing system as shown in FIG. 18, the original color film is subjected to the color separation by the printing plate camera (such as an image scanner or a drum scanner) so as to obtain a plurality of colors required for the color printing (in general, three subtractive mixture primary colors, yellow (Y), magenta (M) and cyan (C) and black (BL), the total being four colors). When the image readout step outputs a digital image data corresponding to each screened dot of each color, the digital image is stored in the storage (MT, hard disk or the like) in the work station.
In the work station, based on the digital image data stored in the storage, the image processing such as layout, color matching, edition and so on is carried out and the result is confirmed on a display device or the output image from the color proof. When the result is acceptable, by a photographic film output apparatus or a recorder of the drum scanner, the color separated films for a block copy is made and sent to the edition step. Based on the color separated films thus obtained, a proof is printed. When the printed proof is unacceptable, the instruction is delivered to the work station so that the above described steps are repeated.
When the proof sheet is OK, based on the color separated films, the planographic printing plate is made and used in the case of the actual printing.
However, the conventional printing system described above with reference to FIG. 18 is fundamentally based on the existence of the color separated films in such a way, the separated color films used in proofing and in making a printing plate and so on, so that the system has the following problems.
The first problem resides in the fact that the image on each of the color separated films takes a long time and a great effort as well and the storage and control of the color separated films also become the problem.
The second problem is that the skilled workers are required in the case of proofing based on the color separated films; that is, no one can easily accomplish such operation. Furthermore, the proofing machine and the printing plate making machine must be installed independently of each other and they are very expensive.
Furthermore, there is a difference in color reproduction and the gain by dots between a printed proof sheet in which the toner images different in color are superposed one upon another in accordance with the above described electrophotographic process on the one hand and a color print obtained by the planographic printing press by using color ink for printing on the other hand. As a result, there is the problem that it is difficult to obtain the proof sheet substantially similar in color to the color print obtained by the planographic printing process.
Moreover there is the problem that the tone reproducibility or tone is different between a proof sheet and a print obtained by the printing press. The reason is as follows. In the case of a proof sheet, one views directly the image of toner deposited over a white sensitive medium such as titanium oxide. On the other hand, in the case of a print obtained by the printing press, it is obtained by the planographic printing process utilizing the difference in the "oil affinity" and the hydrophilic property between the image portions and the non-image portions. The sensitive media and toner used in printing are quite different from those used in printing a proof sheet. As shown in the first quadrant in FIG. 19, in the case of the proof printing, the input-output density characteristic (the gradation reproducibility) may be represented by a linear curve, but in the case of the print, its similar characteristic is not linear.
As shown in the second quadrant in FIG. 19, the tone reproducibility in the case of the planographic printing process is not a correct straight line so that in the case of the output in the actual printing, the tone reproducibility becomes further different. As a result, even when the gradation reproducibility is corrected to as be represented by an ideal straight line in response to the proof sheet, as shown in the third quadrant in the case of the actual printing, the gradation reproducibility is much deviated from a straight line.